Refactor Reportbooru API clients.

* Combine MissedSearchService, PostViewCountService, and
  PopularSearchService into single ReportbooruService class.
* Use Danbooru::Http for these services instead of HTTParty.

test/unit/reportbooru_service_test.rb

@@ -1,8 +1,8 @@
 require 'test_helper'
 
-class PostViewCountServiceTest < ActiveSupport::TestCase
+class ReportbooruServiceTest < ActiveSupport::TestCase
   def setup
-    @service = PostViewCountService.new(reportbooru_server: "http://localhost:1234")
+    @service = ReportbooruService.new(reportbooru_server: "http://localhost:1234")
     @post = create(:post)
     @date = "2000-01-01"
   end